1. Field of the Invention
The present invention relates to a processing device, a processing method, a distributing device and a transmitting system for encoded image-data. Specifically, it relates to the art of producing reduced encoded image-data for a reduction image, using encoded image-data.
2. Description of the Related Art
In recent years, data transmission over a network has become faster, and household appliances such as a personal computer and a digital television receiver have come into wider use. This helps put image-information providing services to practical and broad use. Particularly, such image-information is handled as digital image-data, thus making the quantity of data more massive. This requires operations to be made efficiently, such as image-data storage and transmission. In the field where image-data is handled, there is the art of compressing and encoding image-data. Especially, in the field where static images (or still pictures) are handled, an image compressing-and-encoding art, called the JPEG (Joint Photographic Experts Group), is generally known.
As an example in which such an image compressing-and-encoding art based on the JPEG is in use, there is a field of an electronic still camera which is generally called a digital camera. In the electronic still camera, a photographed image is compressed and encoded into the JPEG form to reduce the quantity of data, and then, it is recorded on a storage medium. In addition, as another example, there is an image distributing system in which image-data is compressed and encoded into the JPEG form and then is distributed. The more advanced a network technology such as the Internet gets, the more popular has this system become.
In the field of an electronic still camera, image-data (hereinafter, called the JPEG image) which is held in the JPEG form inside the camera is generally recorded on a recording medium such as a secure digital (SD) card. In this case, in general, a plurality of JPEG images are also produced as reduced images for printing reference to record them on a recording medium. Such a reduced image is called a thumbnail image. If a plurality of thumbnail images are collected and simultaneously printed out, that printout is called an index print.
To produce this index print, a plurality of JPEG images have to be restored, and then, a thumbnail image needs to be produced for each of them. In other words, restoring a plurality of JPEG images requires a considerable quantity of processing, thereby taking quite a bit of processing time for index printing.
Aiming to resolve this disadvantage, an image processing technology is proposed of producing a thumbnail image at high speed, using image-data which is compressed and encoded (e.g., refer to Patent Document 1: Japanese Patent Laid-Open No. 2000-59612 specification). Patent Document 1 discloses a technology of dividing an original image into blocks called macro-blocks, and producing a thumbnail image by using only a direct-current component coefficient which is included in encoded data that is obtained by encoding a spatial-frequency component of each macro-block. According to this prior art (hereinafter, named Prior Art 1), a thumbnail image is produced using only a direct-current component coefficient. This helps save the time to process an alternating-current component coefficient, or some other time, thereby heightening the speed of processing.
Furthermore, as described above, there is known an image distributing system which uses a network such as the Internet, as another example in which an image compressing-and-encoding technology is used. As such a system, the one that uses a thumbnail image is known. In this image distributing system, a distributor transmits, as an index image used to select an image, thumbnail image-data which corresponds to an image size desired by a user. Then, the user selects a desired image, using this thumbnail image. Next, the user notifies the distributor of a request to distribute the image selected by the user. Thereby, the distributor distributes request contents of a dynamic image (or moving picture) or the like which corresponds to the image selected by the user. Such an image distributing system is practically used in the field of an image distributing system. In this field, especially, users have various desires. Thus, a reduced image such as a thumbnail image is distributed of a variety of sizes. In this case, if the distributed thumbnail image is small, the user can watch a large number of images at the same time. But if it is too small, then the user will find it difficult to view its contents.
Accordingly, a technology is proposed which is capable of confirming a thumbnail image swiftly, and in addition, distributing a thumbnail image of image size desired by the user, if necessary (e.g., refer to Patent Document 2: Japanese Patent Laid-Open No. 58-75390 specification). Patent Document 2 discloses a technology of dividing an original image into macro-blocks, selecting, from among low-frequency component coefficients which are included in encoded data that is obtained by encoding a spatial-frequency component of each macro-block, those coefficients limited in number depending on an image size to be restored, transmitting the same, and producing thumbnail images of the desired sizes at a receiver. More specifically, in this method, an original image is divided into macro-blocks, and encoding conversion processing is executed on an original-image signal which is sampled using a plurality of sampling values of each macro-block. From among a plurality of spatial-frequency coefficients which are obtained in this encoding conversion processing, a plurality of low-frequency component coefficients are selected as one set of coefficients. Herein, the low-frequency component coefficients are fewer than the spatial-frequency coefficients obtained by the encoding conversion processing and include only low-frequency components. The distributor transmits the set of coefficients, for each macro-block, to the receiver. The receiver executes, on this set of coefficients, a decoding conversion which conforms to the size of sample arrangements which correspond to the number of these coefficients. This reproduces an image which has a smaller sampling value than the sampling value of the original image, or a reduced image. This configuration enables transmission time or processing time to be saved as long as the image is allowed to have a small size, according to the prior art (hereinafter, named Prior Art 2) which is disclosed by Patent Document 2. According to Prior Art 2, the larger the reduction rate of an image becomes and the closer it gets to 1, the higher spatial-frequency component coefficients will be and the larger the number of them will be. If required image sizes are large, this prevents the quality of an image from becoming poorer because of a shortage of encoding coefficients.
However, if a relatively large thumbnail image is handled, in Prior Art 1 which uses only a direct-current component coefficient included in encoded data, the image quality becomes poor noticeably. In Prior Art 2, where from among low-frequency component coefficients included in encoded data, a large number of those coefficients suited to a large image size are selected, there is a disadvantage in that processing time or transmission speed is almost on the same level as that of an original image.
In other words, Prior Art 1 uses only a direct-current component coefficient which corresponds to an average picture-element value of each macro-block. Thereby, the larger the image size becomes, the larger a so-called block noise becomes. In Prior Art 2, the larger the image size becomes, the more coefficients are handled and the closer the number of coefficients gets to that of the original image. This lowers processing speed or transmission speed.
Furthermore, in a method of Prior Art 2, an original image is encoded according to a reduction rate every time when it is distributed. This requires encoding for each distribution. As a result, if the number of terminals on the receiver side increases, that makes heavier a load of encoding processing. In other words, encoding has to be executed whenever distribution is conducted. This increases the number of times when encoding processing is executed and makes the processing's load heavier. This processing's load could cause the time from a distribution request to a distribution to be delayed. Furthermore, according to Prior Art 2, an image is held as original-image data which is not compressed and encoded, a storing means having a greater storage capacity is needed than in the case of compressed and encoded image data.